(1) Field of the Invention
The present invention relates to electrophotographic image forming apparatuses and in particular to technology of charging a photoreceptor in a preferable manner by securing a sufficient amount of discharge current without applying high voltage.
(2) Related Art
In an electrophotographic image forming apparatus, it is necessary to uniformly charging the surface of a photoreceptor before forming an electrostatic latent image on the surface of the photoreceptor. Methods of charging can be classified roughly into non-contact charging methods and contact charging methods. Examples of the contact charging methods include a roller charging method and a brush charging method. The roller charging method is a method of charging the surface of the photoreceptor by applying voltage to a charging roller that is in contact with the surface of the photoreceptor.
When the roller charging method is employed, applying an alternating current (AC) voltage allows for more uniform charging onto the photosensitive drum as compared to applying a direct current (DC) voltage only. In particular, if AC bias having a peak-to-peak voltage Vpp that is equal to or greater than twice the difference between the DC voltage and the discharge start voltage Vth is applied, the amount of discharge current increases and leads to stable charging. Note that peak-to-peak voltage Vpp denotes the difference in potential between the maximum voltage Vmax and the minimum voltage Vmin, and the discharge start voltage Vth denotes the voltage that causes discharge to occur between the photoreceptor and the charger.
However, if the discharge current is increased in amount under a high-humidity environment, image deletion, which is caused by adhesion of the product of the discharge, will become more likely to occur. Considering this, there has been a proposal of a charge control method for determining the peak-to-peak voltage Vpp by measuring the amount of current flowing through the charging roller (See Japanese Patent Application Publication No. 2001-201921).
Meanwhile, image forming apparatuses are recently used under a low-temperature and low-humidity environment more frequently. Under a low-temperature and low-humidity environment, the charging device increases in electrical resistance for example, and there is a risk of the occurrence of a charging failure. Even in such a case, it is possible to prevent the occurrence of a charging failure by increasing the peak-to-peak voltage Vpp, because this increases the amount of discharge current. For this reason, there also has been a proposal of an image forming apparatus that changes the peak-to-peak voltage Vpp according to the temperature within the apparatus (See Japanese Patent Application No. 2011-150309).
However, the above-described conventional technologies require an ammeter for measuring the amount of current flowing through the charging roller, and also require a temperature sensor for measuring the temperature within the apparatus. Therefore, in either of the conventional technologies, an increase in parts cost and manufacturing cost is inevitable. Furthermore, since it is necessary to improve the voltage endurance in order to increase the peak-to-peak voltage Vpp, an increase in cost and power consumption of the power supply device for example is also inevitable.
Moreover, the increase in peak-to-peak voltage Vpp also causes problems such as short life as well as the image deletion resulting from the increase in the product of the discharge as described above, because the photoreceptor wears out quickly due to the high voltage.